Natural gas is a hydrocarbon gas mixture consisting primarily of methane gas (CH4) and is generally used as a source of energy. Natural gas can be compressed and transported in gas pipelines but it can also be converted from its primary gas form to a liquid form at cryogenic temperatures for ease of storage and transportation. Liquefied natural gas (LNG) takes considerably less volume than natural gas in a gaseous state. This makes LNG cost efficient to transport over long distances where pipelines do not exist.
Various technologies exist for the production of LNG, especially ones that can be used in industrial base load production plants and in peak shaving plants. These plants generally have large LNG production capacities but they require a substantial upfront investment. For instance, base load production plants often have a LNG production capacity ranging from about 1,500,000 to 5,000,000 MT per year. These plants are generally used to produce large amounts of LNG that will be stored in cryogenic tanks prior to transfer to LNG transport sea vessels or tankers. They are often supplied directly with natural gas from gas wells or from pipelines. Peak shaving plants have LNG production capacities that are often ranging from about 35,000 to 150,000 MT per year. These plants are used for storing natural gas in liquid form to meet the demand of natural gas pipeline during peak consumption periods. They are generally supplied in natural gas of pipeline quality.
Natural gas includes mostly methane in high concentrations, such as about 85% vol. for instance, with the balance of the gas stream including gases such as ethane, propane, higher hydrocarbon components, a small proportion of water vapor, nitrogen and/or carbon dioxide. Other components such as mercury, hydrogen sulfide and mercaptan can also be present in lower concentrations. Variants are possible.
LNG is increasingly used as an alternative fuel for transportation since it offers many advantages over other available kinds of fuel. For instance, it is an alternative fuel cleaner than other fossil fuels, with lower emissions of carbon and lower particulate emissions per equivalent distance traveled. LNG is also generally more efficient and provides a significant increase in the useful life of the engines. However, despite all its advantages, the widespread use of LNG in transportation faces several limitations due in most part to a lack of availability. There is a limited number of LNG production plants and a corresponding limited number of distribution points, i.e. fueling stations, particularly outside densely populated areas. Still, transporting LNG over long distances in relatively small quantities to supply remote fueling stations lowers environmental and economic benefits of LNG.
Small LNG production facilities, often called mini LNG plants, have been suggested in the past. They have LNG production capacities often ranging from about 3,500 to 35,000 MT per year. Mini LNG plants are also generally supplied in natural gas of pipeline quality. They require somewhat lower capital investment costs than base load or peak shaving plants but these costs can still be relatively large compared to their LNG production capacities. They are also less energy efficient than common larger plants. For instance, there is generally a significant loss of natural gas in the order of about 20 to 35% vol. of the initial methane gas input in mini LNG plants. This results in economical losses and releasing such large quantities of methane gas directly into the atmosphere reduces the environmental benefits of LNG in transportation.
Natural gas is only one among a number of different possible sources of methane gas. For instance, landfill sites and anaerobic digesters can generate significant amounts of biogas which contains methane gas, generally in concentration ranging from about 40 to 65% vol. under favorable operating conditions. Other gases that are often present in biogas include carbon dioxide in concentration that can generally reach about 50% vol. of the gas stream, nitrogen in concentration generally varying from a few percent to about 30% vol. of the gas stream, and possibly in smaller concentrations, oxygen in concentration that can generally reach about 3% vol. of the gas stream, and hydrogen sulfide in concentration that can generally reach about 0.5% vol. of the gas stream. These values are only typical examples. Other components can be present in even smaller concentrations, such as siloxanes, mercury, volatile organic carbons (VOC) and mercaptan.
Biogas originating from a landfill site is generally saturated in water at the pressure and temperature conditions occurring at the capture points. Also, it can sometimes have lower methane gas concentrations than the usual amounts due to presence of air infiltrations. If air is introduced directly from external headers, then the concentration of oxygen and nitrogen will substantially remain the same and air will only dilute the biogas generated in the landfill site. However, when air is introduced into the landfill site itself before entering the biogas headers, some or all of the oxygen can be transformed into carbon dioxide while the nitrogen will not be affected.
The methane gas fraction contained in biogas can be transformed into Liquefied Methane Gas (LMG). LMG can provide an equivalent to LNG in terms of quality and energy content. Thus, one could use LMG instead of LNG at fueling stations. This is particularly useful since biogas can be obtained locally, particularly from municipal landfill sites. Transforming biogas into LMG from small distributed production plants would then be highly desirable since this will promote an increase in the number of fueling stations, particularly in remote areas. It can also offer significant environmental and economic benefits over burning biogas in gas flares and/or releasing unburned biogas directly into the atmosphere.
Landfill sites and anaerobic digesters often have a methane production capacity ranging from about 400 to 15,000 MT per year. They are thus smaller in capacity than typical mini LNG plants and the return on investment as well as the profitability of the whole operation may be difficult to obtain using existing approaches. Most liquefaction plants are designed for use in dedicated arrangements that are substantially stable and specific to a given site. Adapting existing designs for use in a wide variety of conditions is not easy to achieve. There are also numerous challenges associated specifically with the transformation of the methane gas fraction contained in biogas into LMG that are unique to biogas. One of these challenges is the unpredictability of the biogas in terms of the flow rate and the proportion of the methane gas fraction, particularly when biogas is captured in a landfill site. The flow rate of biogas collected from a landfill site may sometimes be insufficient to transform it into LMG and/or it may have a methane gas fraction that is insufficient to produce the desired quantity of LMG due to air infiltrations.
Another of the challenges associated with the transformation of the methane gas fraction contained in biogas into LMG is the economics of the whole operation. High capital-investment costs may deter commercial ventures from building a small plant. In particular, the costs cannot be compensated by large volumes of sales. High operational costs of the equipment required to carry out the LMG production will also play an important role. Even when a plant uses its own methane gas it produces for fulfilling its energy requirements, the LMG output will be lower. Yet, losses of methane gas due to limitations in the processes will also have an impact on the profitability of the operation.
A large part of the relatively high capital-investment and operational costs of existing systems are related to the very high pressures involved. Pressures in the order of about 6,800 kPag (1,000 psig), or even more, are not uncommon. They are useful for producing the extremely cold temperatures, i.e. cryogenic temperatures, required for condensing and storing the methane in a liquefied form at about −160° C. However, the acquisition costs of high-pressure compressors and other associated equipment required to build the corresponding plant infrastructure can quickly become a predominant factor, particularly in smaller plants. The energy requirements for operating these high-pressure compressors are also very high.
LNG and natural gas of pipeline quality have both a low nitrogen concentration. Nevertheless, nitrogen can be present in natural gas prior to liquefaction, even after the various gas treatments carried out. For instance, nitrogen is sometimes mixed with natural gas as part of the natural gas extraction process from a gas well. Most of this nitrogen must be removed afterwards, for instance in a distillation column. Cryogenic temperatures are thus useful for separating nitrogen from methane when the concentration of nitrogen is not negligible, for instance about 3% or above.
Nitrogen is generally not considered to be a very good refrigerant but when compressed and then expanded with a very high pressure drop, it can yield very low temperatures and be used as a cryogenic refrigerant to liquefy methane. One approach is to use nitrogen already mixed with the natural gas as a refrigerant to both liquefy the methane gas and separate nitrogen therefrom. U.S. Pat. No. 6,978,638 (Brostow et al.) of 2005 discloses an example of such approach. However, high capital-investment costs, high operational costs and the complexity of such equipment are very limiting factors. Another limitation is that the presence of nitrogen is always needed and the process stops working if the proportion of nitrogen in the gas feed stream becomes too low.
Other existing approaches generally suffer from similar limitations and can be difficult to implement for a number of reasons, particularly in relatively small plants.
Overall, existing approaches are often:                difficult to achieve on relative small implementations, for instance LMG production capacities ranging from about 400 to 15,000 MT per year to match the methane-gas throughput of landfill sites and anaerobic digesters;        not capable of being carried out continuously over extended period of time when the proportion of nitrogen in the incoming gas feed stream falls down to a relatively low concentration;        costly in terms of the upfront investment and energy requirements;        difficult to implement in a wide variety of contexts in order to produce LMG of constant quality regardless of the source of the methane gas being used; and/or        not well adapted for the design of generic plants, such as plants that can be preassembled in a factory and delivered to various kinds of sites as prepackaged units that are ready for operation in a relatively short amount of time.        
Accordingly, there is still room for many improvements in this area of technology.